In continuous casting of thin steel strip, molten metal is cast directly by casting rolls into thin strip. The shape of the thin cast strip is determined by, among other things, the surface of the casting surfaces of the casting rolls.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated laterally positioned casting rolls which are internally cooled, so that metal shells solidify on the moving casting roll surfaces and are brought together at the nip between the casting rolls to produce a thin cast strip product. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a moveable tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the casting rolls so as to restrain the two ends of the casting pool.
The thin cast strip passes downwardly through the nip between the casting rolls and then into a transient path across a guide table to a pinch roll stand. After exiting the pinch roll stand, the thin cast strip passes into and through a hot rolling mill where the geometry (e.g., thickness, profile, flatness) of the strip may be modified in a controlled manner.
The “measured” strip flatness and tension profile as measured at a device downstream of the hot rolling mill are insufficient to control in practice the hot rolling mill because, unlike cold mills (where the measured downstream flatness or tension profile of the strip closely resembles the flatness or tension profile produced off the mill), the flatness or tension profile may differ due to the action of creep. At elevated temperatures, steel undergoes plastic deformation in response to the tension stress at the entry and exit of the rolling mill in the form of creep. The plastic deformation occurring outside the roll gap in the regions where the strip enters and exits the mill causes changes in the entry and exit tension stress profiles, strip flatness as well as strip profile.
The high strip temperature at the exit of steel hot mills also makes difficult the measurement of the strip flatness or tension stress profile by direct contact. Non-contact optical methods for flatness measurement have been used. However, such non-contact flatness measurement results in partial flatness measurement, since at any given time only part of the strip exhibits measured flatness defects. In addition, creep in the strip results in the flatness of the strip at the roll stand exit likely being significantly worse than that measured downstream at practical flatness gauge locations.
In twin roll casting of thin strip, the cast strip is thinner than typically found in traditional strip in hot mills. Typically in twin roll casting, the thin strip is cast at a thickness of about 1.8 to 1.6 mm and rolled to a thickness between 1.4 and 0.8 mm. The strip entry temperature to the hot mill is higher than found in the final stand of the typical hot mill, approximately 1100° C. A consequence of thin strip high temperature and casting process is that the strip entry tension is low, and therefore is more susceptible to buckling and creep prior to entry into the hot mill. In addition, in thin strip casting, it is desirable to produce strip of a desired strip profile while maintaining acceptable flatness, since the product may be used as cold rolled replacement. The strip geometry is largely controlled by the caster. Low tensions employed in hot rolling mills results in small local roll-gap errors and loss of tension stress at points across the strip width, and results in strip buckles and poor strip flatness. We have found that tension stress provides a way to control the strip flatness.
A method is disclosed for controlling strip geometry in casting strip having a hot rolling mill comprising the steps of:
measuring an entry thickness profile of an incoming metal strip before the metal strip enters the hot rolling mill;
calculating a target thickness profile as a function of the measured entry thickness profile while satisfying profile and flatness operating requirements;
measuring an exit thickness profile of the metal strip after the metal strip exits the hot rolling mill;
calculating a differential strain feed back from longitudinal strain in the strip by comparing the exit thickness profile with the target thickness profile derived from the measured entry thickness profile; and
controlling a device capable of affecting the geometry of the strip exiting the hot rolling mill in response to at least the differential strain feed-back.
The method of controlling strip geometry in casting strip having a hot rolling mill may further comprise the steps of:
calculating a roll gap pressure profile from the entry thickness profile and dimensions and characteristics of the hot rolling mill;
calculating a feed-forward control reference and/or a sensitivity vector as a function of the target thickness profile and the roll gap pressure profile to allow compensation for profile and flatness fluctuations in the cast strip; and
further controlling the device capable of affecting the geometry of the strip exiting the hot rolling mill in response to the calculated feed-forward control reference and/or the calculated sensitivity vector.
The profile and flatness operating requirements may be selected so that the target thickness profile inhibits strip buckling. The device capable of affecting the geometry of the strip exiting the hot rolling mill may be selected from one or more of the group consisting of a bending controller, a gap controller, a coolant controller, and other devices capable of modifying the loaded roll gap of the hot rolling mill.
The method of controlling strip geometry in casting strip having a hot rolling mill may further comprise the step of generating an adaptive roll gap error vector from the measured exit thickness profile and using the adaptive roll gap error vector in calculating at least one of the feed-forward control reference and the sensitivity vector.
The method of controlling strip geometry in casting strip having a hot rolling mill may further include calculating the target thickness profile by performing at least one of time filtering and spatial frequency filtering.
The method of controlling strip geometry in casting strip having a hot rolling mill may also have the controlling step include performing symmetric feed-back control and asymmetric feed-back control of the bending controller and the gap controller. The controlling step may alternatively, or in addition, include subtracting out systematic measurement errors from the differential strain feed back when the rolling mill is engaged, the systematic measurement errors being generated through comparison of the entry and exit thickness profiles when the rolling mill is disengaged. The controlling step may also include performing temperature compensation and buckle detection, or performing at least one of operator-induced coolant trimming and operator-induced bending trimming.
More particularly, the method for controlling strip geometry in casting strip having a hot rolling mill may be used in continuous casting by twin roll caster comprising the following steps:
(a) assembling a thin strip caster having a pair of casting rolls having a nip therebetween;
(b) assembling a metal delivery system capable of forming a casting pool between the casting rolls above the nip with side dams adjacent the ends of the nip to confine the casting pool;
(c) assembling, adjacent the thin strip caster, a hot rolling mill having work rolls with work surfaces forming a roll gap between them through which incoming hot strip is rolled, the work rolls having work roll surfaces relating to a desired shape across the work rolls:
(d) assembling a device capable of affecting the geometry of the strip exiting the hot rolling mill in response to control signals;
(e) assembling a control system capable of calculating a differential strain feed-back from longitudinal strain in the strip by comparing a exit thickness profile with a target thickness profile derived from a measured entry thickness profile, and generating control signals in response to the calculated differential strain feed-back;
(f) connecting the control system to the device capable of affecting the geometry of the strip exiting the hot rolling mill in response to the generated control signals from the control system.
To perform the method in a twin roll caster molten steel may be introduced between the pair of casting rolls to form a casting pool supported on casting surfaces of the casting rolls confined by the side dams, and the casting rolls counter-rotated to form solidified metal shells on the surfaces of the casting rolls and cast thin steel strip through the nip between the casting rolls from the solidified shells. The device affecting the geometry of the strip being processed by the hot rolling mill may be capable of varying the roll gap of the work rolls, bending by the work rolls, and/or coolant provided to the work rolls in response to at least one of the control signals, to affect the geometry of the hot strip exiting the hot rolling mill.
Also disclosed is a control architecture for controlling strip geometry in casting strip having a hot rolling mill comprising:
an entry gauge apparatus capable of measuring an entry thickness profile of an incoming metal strip before the metal strip enters the rolling mill;
a target thickness profile model capable of calculating a target thickness profile as a function of the measured entry thickness profile while satisfying profile and flatness operating requirements;
an exit gauge apparatus capable of measuring an exit thickness profile of the metal strip after the metal strip exits the rolling mill;
a differential strain feed back model capable of calculating a differential strain feed-back from longitudinal strain in the strip by comparing the exit thickness profile with the target thickness profile derived from the measured entry thickness profile; and
a control model capable of controlling a device capable of affecting the geometry of the strip exiting the hot rolling mill in response to the differential strain feed back.
The target thickness profile model may inhibit strip buckling. The differential strain feed back model may also include temperature compensation capability and buckle detection capability. The differential strain feed back model further may include an automatic nulling capability capable of subtracting out systematic errors from the differential strain feed back when the rolling mill is engaged, the systematic errors being generated through comparison of the entry and exit thickness profiles when the rolling mill is disengaged.
The control architecture for controlling strip geometry in casting strip having a hot rolling mill may further comprise:
a roll-gap model capable of calculating a roll gap pressure profile from the entry thickness profile and dimensions and characteristics of the hot rolling mill, and
a feed-forward roll stack deflection model capable of calculating a feed-forward control reference and/or a sensitivity vector as a function of the target thickness profile and the roll gap pressure profile to allow compensation for profile and flatness fluctuations in the cast strip.
The adaptive roll stack deflection model may be capable of generating an adaptive roll gap error vector from the measured exit thickness profile and using the adaptive roll gap error vector in calculating at least one of the feed-forward control reference and the sensitivity vector. The target thickness profile model may further include at least one of time filtering capability and spatial frequency filtering capability as part of calculating the target thickness profile. The control model may include a symmetric feed back capability and an asymmetric feed back capability for controlling the bending controller and the gap controller.
Again, the device capable of affecting the geometry of the strip exiting the hot rolling mill may be selected from one or more of the group consisting of a bending controller, a gap controller, and a coolant controller. The control architecture may also support at least one of operator-induced coolant trimming and operator-induced bending trimming.
The control architecture may be provided in a thin cast strip plant for continuously producing thin cast strip to controlled strip geometry which comprises:
(a) a thin strip caster having a pair of casting rolls having a nip therebetween;
(b) a metal delivery system capable of forming a casting pool between the casting rolls above the nip with side dams adjacent the ends of the nip to confine the casting pool;
(c) a drive capable of counter-rotating the casting rolls to form solidified metal shells on the surfaces of the casting rolls and cast thin steel strip through the nip between the casting rolls from the solidified shells;
(d) a hot rolling mill having work rolls with work surfaces forming a roll gap between through which cast strip from the thin strip caster may be rolled;
(e) a device connected to the hot rolling mill capable of affecting the geometry of the strip processed by the hot rolling mill in response to control signals; and
(f) a control system capable of calculating a differential strain feed-back from longitudinal strain in the strip by comparing an exit thickness profile with a target thickness profile derived from a measured entry thickness profile, capable of generating the control signals in response the differential strain feed-back, and connected to the device to cause the device to affect the geometry of strip processed by the hot rolling mill in response to the control signals.
In the thin cast strip plant for producing thin cast strip with a controlled strip geometry by continuous casting, the control system may further be capable of calculating a feed-forward control reference and a sensitivity vector, and further capable of generating the control signals the feed-forward control reference, and the sensitivity vector. The feed-forward control reference and the sensitivity vector are calculated as a function of a target thickness profile, derived from a measured entry thickness profile, and a roll gap pressure profile to allow compensation for profile and flatness fluctuations in the cast strip.
These and other advantages and novel features of the present invention, as well as details of illustrated embodiments thereof, will be more fully understood from the following description and drawings.